The invention relates generally to desiccants for drying fluids containing fluorinated propenes. In particular, the invention relates to the use of desiccants comprising molecular sieves having defined pore sizes.
It is generally recognised that it is important to control the levels of water present within vapour compression refrigeration systems that utilise halogenated fluids as the heat transfer media. High levels of moisture in such systems can result in a number of reliability and performance problems. With levels approaching or above those where free-phase water can form, solid clathrates or ice crystals can form. Solid clathrates can form at temperatures above that of the normal freezing point of water. These solid materials can act to restrict refrigerant flow, particularly through the expansion device, typically a valve, orifice tube or capillary tube, that is used to regulate refrigerant flow through the system.
At lower levels of moisture, many polymeric materials found in hermetic compressor systems, in particular those associated with the hermetic electric motor insulation such as nylon and PET, may be subject to hydrolytic degradation leading to motor burn-out and premature system failure. Moisture may also act to corrode metallic components of the system and contribute to the phenomenon of copper-plating where copper is transported from components constructed of copper and deposited onto ferrous-alloy surfaces. When these surfaces are in the compressor, such as valves and piston elements, this deposition acts to reduce mechanical clearances and may eventually lead to seizure.
In order to minimise the detrimental effects of moisture in refrigeration systems, these systems incorporate a dryer material in order to selectively absorb moisture from the circulating fluid. Traditionally these driers have been manufactured from a number of materials including activated alumina, silica gel and aluminosilicate molecular sieves (zeolites). The desiccant is usually used in the form of a porous moulded core consisting one or more of the desiccant materials, or in the form of a loose-fill of beads or pellets of desiccant. In either case, the desiccant is held within a cartridge and the refrigerant, in liquid or vapour form, is caused to pass through the cartridge in contact with the desiccant.
The zeolite molecular sieves are of particular interest since they can combine a high capacity for moisture retention with the capability of reducing the moisture content of the refrigeration fluid to low levels. In order to achieve satisfactory desiccant performance with such molecular sieves, it is important to minimise the competitive absorption of refrigerant. This is normally achieved by selecting a molecular sieve with a pore-opening dimension that is sufficiently small such that refrigerant absorption is minimised but which is sufficiently wide to maintain satisfactory rates of moisture absorption. This minimisation of refrigerant absorption is also required in order to avoid degradation of the refrigerant through reaction within the molecular sieve channels.
Whilst moisture control within a refrigeration system is clearly important, it is also necessary to dry refrigerant fluids as part of their manufacturing process. In this way, any moisture that may be incorporated into the refrigerant through steps of the manufacturing process, e.g. by aqueous scrubbing, may be removed before the fluid is packaged or placed in receiver vessels. Drying the fluid thus avoids any corrosion or icing issues associated with handling and storing the fluids and helps to ensure that subsequent introduction of the fluid into a refrigeration circuit does not introduce excessive levels of moisture that may overload the drying capacity of any in-line system desiccant.
The Montreal Protocol brought about the replacement of the traditional chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) refrigerants including R-12 (dichlorodifluoromethane), R-22 (chlorodifluoro-methane) and R-502, an azeotropic mixture of R-22 and R-115 (chloropentafluoroethane). These substances have generally been replaced by hydrofluorocarbon (HFC) refrigerants of general formula CnHxFy (where n=1 to 3, x=1 to (2n−1) and (x+y)=2n+2) and their mixtures.
These refrigeration fluids are generally used in combination with a synthetic compressor lubricant, most often an ester, a polyalkyleneglycol (PAG) or a polyvinyl ether (PVE). Both the HFC refrigerants and their synthetic lubricants are more polar and more hygroscopic than the traditional combination of CFC and HCFC refrigerants with their mineral oil or synthetic hydrocarbon compressor lubricants.
One of the most common refrigerants used historically was R-12. This was replaced by R-134a (1,1,1,2-tetrafluoroethane) in the 1990s in the majority of its application areas. One of the single largest application areas for R-134a is in automotive air conditioning systems where it is used in conjunction with a polyalkylene glycol compressor lubricant. Other application areas include domestic and industrial refrigeration where it is generally used in conjunction with an ester or PVE compressor lubricant, typically an ester from the family of neopentylpolyol esters (POEs). R-134a is also a component of refrigerant blends that have been used to replace R-22 and R-502 in commercial and residential freezing, refrigeration and air conditioning applications.
The global warming potential (GWP) of R-134a, 1300 relative to CO2 on a 100-year timescale, has led to pressure to develop alternative ozone-benign refrigerants having a reduced GWP. One family of fluids under consideration as alternatives to R-134a is the fluorinated propenes. Of particular interest is R-1234yf (2,3,3,3-tetrafluoropropene) which may be used on its own, or blended with other low GWP fluids such as CFI, to produce non-flammable mixtures with appropriate physical and thermophysical properties to be used in a number of the applications in which R-134a is currently used. Also of interest are refrigerant fluids comprising R-1225ye (1,1,1,2,3 pentafluoropropene), preferably absent CF3I. Of interest are both isomeric forms of R-1225ye, namely the E and Z forms. R-1234yf, like the established HFCs, is a polar species and may be used in conjunction with PAG, PVE and ester lubricants currently used with R-134a or other HFC refrigerants. CF3I is relatively non-polar and may be used with traditional mineral oil or synthetic hydrocarbon lubricants. Blends comprising R-1234yf and CF3I may be used with PAG, PVE or ester lubricants or with traditional mineral oil or synthetic hydrocarbon lubricants.
There is therefore a need to provide a compatible desiccant material for use with R-1234yf or R-1225ye or with blends comprising R-1234yf and CF3I. Further, there is a need to provide a desiccant that is compatible with R-1234yf, R-1225ye and blends containing R-1234yf and CF3I, and an associated compressor lubricant.
According to the present invention, there is provided a method of drying a fluid comprising a fluoropropene, which method comprises the step of contacting the fluid with a desiccant comprising a molecular sieve having openings which have a size across their largest dimension of from about 3 Å to about 5 Å.
Preferably, the molecular sieve has openings which have a size across their largest dimension of from about 3 Å to about 4 Å.
Conveniently, the molecular sieve has openings which have a size across their largest dimension of about 4 Å.
Advantageously, the fluoropropene is R-1234yf, or R-1225ye, for example R-1225ye either in the thermodynamic or kinetic equilibrium blend of E and Z isomers, or relatively pure individual isomeric forms, e.g. greater than about 50%, conveniently greater than about 80%, conveniently greater than 90%, of the Z isomer, or greater than 50%, conveniently greater than 80%, conveniently greater than 90% E isomer.
Preferably, the fluid comprises at least one additional refrigerant component.
Conveniently, the at least one additional refrigerant component is selected from CF3I, R-134a and R152a.
Advantageously, the fluid further comprises a lubricant.
Preferably, the lubricant is selected from esters, PAGs, PVEs, mineral oils and synthetic hydrocarbons.
Conveniently, the fluid further comprises a stabiliser.
Advantageously, the stabiliser is selected from epoxides, dienes and phenols.
Preferably, the fluid further comprises a dye.
Conveniently, the dye is a fluorescene.
Advantageously, the desiccant comprises at least one further desiccant or adsorbent other than the molecular sieve.
Preferably, the at least one further desiccant or adsorbent is selected from alumina, silica and activated carbon.
Conveniently, the desiccant does not contain any further desiccant other than the molecular sieve.
Advantageously, the fluid is a heat transfer fluid.
Preferably, the desiccant is contained in a cartridge.
According to a further aspect of the invention, there is provided a method of manufacturing a fluid comprising a fluoropropene, which method comprises a method of drying the fluid as defined herein.
According to another aspect of the invention, there is provided a method of providing cooling using a heat transfer fluid comprising a fluoropropene, which method comprises a method of drying the fluid as defined herein.
Preferably, the method of providing cooling is performed in a mobile air conditioning system.
Conveniently, the mobile air conditioning system is an automotive air conditioning system.
According to a further aspect of the invention, there is provided a heat transfer device comprising a heat transfer fluid comprising a fluoropropene, and a desiccant comprising a molecular sieve having openings which have a size across their largest dimension of from about 3 Å to about 5 Å.
Preferably, the heat transfer device is a refrigeration system.
Conveniently, the heat transfer device is an automotive air conditioning system.